1. Field of the Invention
The present invention relates to a shaped glazing provided with a screen process printed current or electrical network for deicing and demisting. It relates in particular to a glazing of a car rear window type. More specifically, the invention relates to a glazing having a relatively pronounced curvature along its edges parallel to the collecting strips to which converge all the lines of the heating network and which are connected to current leads.
2. Description of the Background Art
Numerous cars are nowadays provided with a heated rear window. According to FR-B-1 464 586 such a glazing is obtained by depositing on the surface of a glass sheet a series of narrow resistance strips having an electrically conductive composition. These resistance strips are screen process printed flat on the glass sheet prior to its cambering, so that the baking of the electrically conductive composition takes place during the heating preceding the cambering and tempering of the glazing. The electrically conductive composition is formed by a pasty suspension in an organic binder of metallic silver and a frit, i.e., a low melting point glass. The resistance strips forming the network are relatively thin, so as not to interfere with the visibility through the glazing. They issue onto wider and therefore more conductive collective strips at positions close to the edges of the glazing. These collecting strips are formed from a composition identical to that of the resistance strips and are deposited in the same way and preferably at the same time as the latter. The current leads are then welded to these collecting strips.
With a view to the fitting to the vehicle by bonding or adhesion, which presupposes an opaque peripheral strip masking the adhesive from the outside and protecting it against solar radiation, or simply for aesthetic reasons, the glass sheet is usually provided with a black enamelled frame. The latter is obtained by screen process printing deposition of a paste relatively similar to a silver paste, but in which the silver is replaced by the appropriate dyes. Thus, prior to its cambering, the glazing undergoes at least two screen process printing operations, namely that corresponding to the deposition of the framing layer and, after drying, that corresponding to the heating network. To these may possibly be added a third screen printing step for locally increasing the layer thickness, e.g., in the areas where the leads are welded, and generally for adjusting the thickness of the network and consequently its conductivity.
The glazing can then be passed to the cambering/tempering installation, where it is heated to the cambering temperature and is shaped in a cambering station. The manner of such shaping will not be described in detail, because numerous processes already exist and are well known. However, the shapes which are most difficult to obtain, at least when respecting the most severe optical quality standards, are those having surfaces with a very small radius of curvature with the fold lines located in the marginal areas of the glass sheet. However, it is precisely these shapes which are required in the case of car rear windows. Moreover, the need for an appropriate optical quality leads to a very marked preference for cambering processes in which the central area of the glazing, through which the driver sees, is not pressed between two cambering molds. Thus, in most cases, only the periphery of the glazing is strictly in accordance with a predefined template, and outside said periphery there remains a certain contour imprecision.
As a rule, these slight contour variations cause no problems. But they can still be prejudicial in the folded marginal zone constituting the engagement zone of the glazing on the vehicle body. Curvature defects can then lead to a certain sealing inefficiency, which produces a tendency to leak. In extreme cases, these defects may even make it impossible to fit the rear window in its frame. These cambering defects may also make it impossible to "slide" the glazing towards its final position (in case of glazings partly covering the body).
It is known that the contour of a glazing is very closely dependent on the temperature profile of the glass on leaving the furnace, the deformation speed of the glass increasing as the latter becomes hot. At the temperatures involved here, a difference of a few degrees is sufficient to produce cambering defects of several tenths of a millimeter, i.e., precisely those which are to be avoided here.
Different means have been proposed for controlling as accurately as possible the heating profile of a glass sheet in a heating furnace. However, these means, e.g., in which a burner accompanies the glass sheet in its advance in the furnace, make the installation more complicated, make numerous settings necessary and in any case the results obtained are not always completely satisfactory with respect to the reproducibility of the shapes obtained.